1. Field of the Invention
This invention relates generally to casting devices for making investment molds and, more particularly, to a separable segmented casting ring for making investment molds for casting of dental prosthesis.
2. Background Art
As used herein the following terms have the following meanings.
A “casting ring”, also known as a “pressing ring” is a hollow, typically cylindrical or oval casing or sleeve that surrounds the wax pattern of the dental appliance to be molded. It is attached to a base known as a “former base” to form a vessel or container into which the investment solution is poured. The investment material then hardens around the wax pattern to form the “investment mold”. The present invention is directed to this type of ring.
An “investment mold”, also known as an “investment ring” or “mold”, is the generally cylindrical or oval ring comprised of the investment material that encases the wax pattern of the dental appliance and which is heated to evacuate the wax and leave a cavity that is an exact negative of the dental appliance to be molded.
In dental practice “investment casting”, also known as “precision casting”, or the “lost wax” technique, is used to create a casting from a wax pattern that is a duplicate of the object to be cast. The wax pattern is an accurate replica of the shape of a dental appliance. The wax pattern is attached to one or more thin wax sprues. The sprue is attached to a cylindrical or conical sprue former in a base to hold the wax pattern in place. A hollow, typically cylindrical or oval casing or sleeve known as “casting ring” or “pressing ring” is attached to the former base and surrounds the sprues and wax pattern of the dental appliance to be molded and form a vessel or container into which the investment solution is poured. The investment material then hardens around the wax pattern to form the investment mold.
When the base of the sprue former is removed from the investment mold, it leaves a cylindrical or conical shaped cavity in the end of the investment mold with the wax sprue wires extending from the cylindrical or conical shaped cavity into the wax pattern of the dental appliance to be molded.
After the former base has been removed from the investment mold and the casting ring, the casting ring is removed from the mold. In the case of cylindrical rings and molds, this is typically accomplished by manually pressing the mold out of the ring by applying pressure with the thumbs to force the mold out from one end of the ring.
The investment mold is then heated in an oven to melt and burn out the wax, and remove or divest the wax material and cure the mold, leaving a cavity that is an exact negative of the dental appliance to be molded connected by sprue channels to the cylindrical or conical shaped cavity in the end of the investment mold.
In a conventional hot pressing process, the investment mold is inverted and placed in a hot press oven and a disk or ingot of the dental material such as ceramic dental material (generally referred to as a pressable ceramic) that is to be used to form the appliance is placed in the cylindrical or conical shaped cavity in the end of the mold and heated until it is fluid or semi-fluid and is injected under pressure into the cavity of the mold through the sprue channels by means of a pressing plunger.
One of the problems associated with conventional cylindrical casting rings and pressing rings in a standard smaller diameter size is that typically no more than two dental appliances will fit in the interior, otherwise the investment mold will crack when subjected to the piston pressure in the pressing oven because of the small space between the inside diameter of the ring and the outermost dental pattern cavity. Typically, this problem is solved by using a larger diameter ring to form to mold having a greater diameter. However this solution also presents several problems, such as: maintaining an inventory of different size rings; the amount of investment material used, and wasted, is greater; the larger diameter mold requires more heat and pressing time, resulting in production costs; and more time and labor is required to divesting the dental appliance from the mold.
Another problem associated with the conventional mold forming process is that an exothermic reaction occurs during hardening of the investment material into an investment mold. This occurs at several stages: (1) setting, when the investment material hardens into a solid; (2) curing, additional hardening of the investment mold; and (3) during the pressing operation when the molten dental material is pressed into the mold by a plunger. The investment mold has a smaller volume of investment material and a smaller cross sectional area surrounding the wax pattern of the dental appliance and the cavity formed thereby, thus, the location of the greatest heat buildup due to exothermic reaction and stress as the hot dental material is injected under pressure into the cavity of the investment mold occurs in the area surrounding the wax pattern of the dental appliance and the cavity formed thereby.
The heat generated by both setting and curing causes expansion of the investment material which, when constrained by the casting assembly (casting rings and sprue formers), may result in compression on, and distortion of, the wax pattern within the investment material, or cracking of the mold. Cracking of the mold can also occur due to the heat and pressure that occurs during the pressing operation when the molten dental material is pressed into the mold by a plunger.
There are several patents that disclose casting rings of various constructions that attempt to solve problems associated with investment molds for casting of dental prosthesis.
Carpenter, U.S. Pat. No. 1,976,655, discloses an investment adapted to be encompassed by a casting ring and provided with a casting cavity and a sprue leading thereto. The investment is formed at the upper end portion thereof with a gate of flared contour, the base of the gate being disposed axially of the investment and in the form of a concavity opening centrally thereof into the sprue. The portion of the gate above the concavity is formed with a plurality of endless horizontally disposed shoulders inclining downwardly towards the axis of the investment and a plurality of superposed endless risers inclining from their lower ends in a direction away from the axis of the investment and connecting the inner side of an upper shoulder to the outer side of a lower shoulder, the lowermost shoulder merging at its inner side into the upper end of the wall of the concavity, and the uppermost shoulder being positioned at the terminus of the end portion.
Cooper, U.S. Pat. No. 4,161,208 discloses a precision investment casting assembly having a model support consisting of a central upstanding sprue former and outwardly extending tubes to support wax models. The sprue former is held at one end by a resilient base. A somewhat key-hole shaped recess in the base receives a split flask which encircles the support. An elongated pin engages the free edges of the split flask to close it while embedment material is poured around the support. The pin is removed to release the flask from the embedment material after casting. Proper orientation of the support elements is aided by a plurality of keys and keyways.
Rousseau, U.S. Pat. No. 4,508,155 discloses an apparatus for obtaining improved dental castings comprising an expandable investment ring having a removable base. Formed on the top surface of the base, and within the void defined by the ring, is an indexed sprue former. A curved runner bar having an indexed coping is formed at one end thereof in mating relation to the sprue former. The investment ring is defined by a substantially columnar tube open at each end thereof and including a fracture along its entire longitudinal dimension formed along a zig-zag path. The fracture is initially sealed with a wax prior to pouring the investment material, and then as the investment material hardens and releases heat, the entire ring may expand.
Rousseau, U.S. Pat. No. 4,749,020 discloses an apparatus for the preparation of a dental casting from a dental pattern utilizing an exothermic investment material which expands upon curing which comprises a base having a top surface for supporting the dental pattern and a container having a first and a second open end. The container and the base partially enclose the dental pattern when the first open end of the container is disposed upon the top surface of the base. The second end of the container enables the introduction of the exothermic investment material to completely encompass the dental pattern. The container comprises a first and a second container portion with each of the first and second container portions having a first and a second terminating edge. The first and second terminating edges of the first container portion respectively cooperate with the second and first terminating edges of the second container portion for retaining the uncured exothermic investment material within the container. The first and second terminating edges of the first container portion separate from the second and first terminating edges of the second container portion during the expansion of the exothermic investment material upon the exothermic reaction thereof for producing improved accuracy in the dental casting.
Berger, et al., U.S. Pat. No. 5,406,999, discloses a ringless casting oval for making investment molds for precision casting which has an oval casing which fictionally engages an oval base. Anti-rotation tabs of the base extend into anti-rotation slots in the casing, with both casing and base made of flexible plastic material.
Sullivan, et al., U.S. Pat. No. 7,114,547 discloses an apparatus comprising a casting ring and base sized and dimensioned to be coupled together to form a cavity having one or more walls comprising an inner surface of the casting ring, having a bottom comprising an upper surface of the base, and having a form receiving member portion of the base extending into the cavity. This embodiment includes an indicator forming portion sized and positioned to form an indicator on a mold produced by at least partially filling the cavity with investment and allowing the investment to harden. In some instances the casting ring may have a lower segment sized and dimensioned to surround and receive an upper segment of the base to couple the ring and base together, wherein, the casting ring latches onto an outwardly protruding shoulder of the base when the base and casting ring are coupled together.
Helmberger, et al., U.S. Pat. No. 7,507,080, discloses an apparatus for manufacturing muffles for producing dental prosthetic parts comprises a collar divided at least once in the peripheral direction to form a receiving space for an investment material in which at least one pattern for a dental prosthetic part is arranged before the filling with the investment material. At least one pattern holder simultaneously serves as a spacer for an injection plunger passage and a base part closes an open side of the collar. Holding means encompasses the collar to hold the collar together, with the collar being conical with respect to the longitudinal axis thereof at least in one section for simpler manufacture and better handling. The holding means includes at least one ring, in particular a rigid ring, which has a conical inner surface with which it is pushed onto the conical section in the collar.